1. Field of the Invention
This invention relates generally to medical instrumentation and more particularly to opto-electronic devices that non-invasively measure and determine the velocity and magnitude of blood flow within tissues as well as the thickness and dynamics (other than and/or unrelated to blood flow) of specific components of the tissue when it is comprised of distinct layers. Moreover, it also pertains to measurement instruments that possess the ability to determine the precise location of boundary regions between distinct layers of tissue, as well as triangulate onto the region(s) where initial fluid movement occurs.
2. Description of the Prior Art
Laser-Doppler methodology, as applied to the measurement of blood flow, was initially described by in an article entitled "In Vivo Evaluation of Microcirculation by Coherent Light Scattering", written by Stern, which appeared in Nature, Vol. 254, pages 56-58 in 1975. The basic principles of this methodology are that the frequency of the laser light back scattered or reflected by a laser illuminated tissue is Doppler broadened, due mostly in part to the motions of red blood cells. The extent to which the initial laser frequency is broadened as well as the light intensity within specific parts of the broadened frequency spectrum, both provide information regarding the relative motions and concentration of red blood cells within the tissue.
Utilizing these established methodologies which correlate laser-Doppler signals to red blood cell concentrations and motions within tissues, several commercially-available blood flow monitors that rely on laser/laser-diode and fiber optic technologies have been developed. The essential elements of these systems incorporating the laser-Doppler methodology are: a) a laser beam delivered directly onto a part of the tissue, usually by means of an optical "outlet" fiber where the laser light is unfocused and divergent; b) an optical element that receives a portion of any back scattered or reflected laser light from the illuminated tissue, also typically by means of an optical "pick-up" fiber placed in the vicinity of the outlet fiber (.gtoreq.100 .mu.m optical fiber center-center distance); c) an appropriate means for converting photon flux to electrical signals, and d) an algorithm that realistically represents the movement and approximate numbers of red blood cells within the tissue, as a function of time. A characteristic common to these first-generation blood flow monitoring systems is that they all use outlet and pick-up optical fibers that directly contact the tissue when delivering and accepting laser light normal to a surface of the skin being measured.
More recently, second generation blood flow monitors have incorporated new adaptations in the optics of the laser light delivery system and back scattered laser light detection technologies, which now allow for the measurement of blood flow over an area of a body part. In particular, U.S. Pat. No. 4,862,894 issued to Fuji on Sep. 5, 1989; U.S. Pat. No. 5,291,885 issued to Taniji, et al. on Mar. 8, 1994; U.S. Pat. No. 5,291,886 issued to Katayama, et al. on Mar. 8, 1994; and U.S. Pat. No. 5,339,817 issued to Nilsson on Aug. 23, 1994, all describe systems which permit the measurement of blood flow within a body part.
The optical modification taught by these systems is comprised of a laser light source that can be focused, to a point or a line source, and positioned at specific locations within an area of body by means of scanning optical mirrors. The total back scattered or reflected laser light Doppler or speckle signal from each independent location within an area of body is also collected by the same optical mirrors and partially reflected to a photodetecting system. The obtained electrical signals, in time, are then processed in a manner similar to the first generation blood flow monitors so as to represent the movement and approximate numbers of red blood cells at each specific location within an area of the body. Upon completion of one scanning location, the laser beam is repositioned to a new location, where the Doppler signal collection and analyses, in time, is once again performed. This process is repeated for the entire area of the body that is of interest. Concurrent with obtaining blood flow characteristics at each location within an area of body, a visible image of the area is also recorded by means of video imaging. Both the visible image and blood flow characteristics at each location are later superimposed to yield the approximate characteristics of blood flow for the area of the body. An additional feature of the new optical arrangement is that it is no longer in direct contact with the tissue. To its detriment, however, is that the delivery and acceptance of laser light is still essentially normal to the skin surface.
Regardless of which type of prior art optical arrangement one employs however, systems employing these prior art arrangements are inadequate in locating precisely the region within the skin where blood flow originates. In particular, of four main criteria established to determine the utility of systems to measure blood flow within the skin: 1) non-traumatic and non-invasive, 2) accurately follow blood flow, 3) sensitive to small changes, and 4) insensitive to blood flow from other parts of the tissue or tissues, all blood flow monitors described above fail to meet two of the most significant criteria. Specifically, they are insensitive to small changes and insensitive to blood flow from other parts of the tissue or tissues. Moreover, these laser Doppler systems also fail to yield other characteristics of the skin apart from blood flow. Consequently, a continuing need exists in the art for methods and apparatus which provide non-invasive blood flow and tissue information.